1. Field of the Invention
The invention relates to a control apparatus and a control method for a four-stroke premixed compression ignition internal combustion engine that operates with high fuel efficiency when the load on the engine is high.
2. Background of the Invention
As the conventional operation mode of an internal combustion engine, a premixed compression ignition mode is known. In the premixed compression ignition mode, fuel-air charge generated in a combustion chamber is compressed so that the fuel-air charge autoignites, and burns in an extremely short period. In this mode, burned gas is generally added to the fuel-air charge to increase the temperature of the fuel-air charge.
Japanese Patent Application Publication No. JP-A-11-264319 describes a control apparatus that discharges the burned gas to an exhaust port from a combustion chamber by opening an exhaust valve during exhaust stroke, and then reintroduces the burned gas into the combustion chamber from the exhaust port by reopening the exhaust valve during intake stroke.
As the output required from the engine increases (i.e., the engine load increases), the amount of air required for combustion increases. The above-described control apparatus fixes a timing at which the exhaust valve reopens (i.e., exhaust-valve reopen timing) during the intake stroke. By reopening the exhaust valve at the fixed timing, burned gas in the amount required for combustion is introduced into the combustion chamber when the load on the engine is low. As the required output increases, the control apparatus decreases the lift of the exhaust valve when the exhaust valve reopens during the intake stroke, and decreases the cross sectional area of the exhaust port through which the burned gas is introduced into the combustion chamber. Thus, as the required output increases, the amount of burned gas introduced into the combustion chamber decreases. Accordingly, when the load on the engine is high, a sufficient amount of air is introduced into the combustion chamber.
The control apparatus reopens the exhaust valve to introduce the burned gas into the combustion chamber before a great amount of air is introduced into the combustion chamber. When the exhaust valve reopens, the pressure of the burned gas in the exhaust port is extremely high. Therefore, even if the lift of the exhaust valve (or the cross sectional area of the exhaust port) is decreased, the amount of burned gas may not be sufficiently decreased. In other words, when the amount of burned gas needs to be sufficiently decreased, the lift of the exhaust valve must be decreased to an extremely small value. Therefore, there is a possibility that a sufficient amount of air is not introduced into the combustion chamber when the load on the internal combustion engine is high.
If the lift of the exhaust valve is decreased to an extremely small value, the pumping loss is increased due to the resistance against the burned gas introduced into the combustion chamber from the exhaust port. Accordingly, it is difficult for the control apparatus to operate the internal combustion engine with high fuel efficiency while introducing as much air as required for combustion into the combustion chamber.